Cystatin C is produced as cysteine protease by nucleated cells throughout the body. Cystatin C belongs to the cystatin superfamily and is a basic low-molecular-weight protein having a molecular weight of 13 kD. In clinical tests, it has been drawing attention as a marker of renal function to replace creatinine (see Non-patent Document 1). In clinical tests, since a large number of specimens are handled at once, an automated accurate means for quantifying cystatin C is required.
As a method of quantifying cystatin C, for example, a latex turbidimetric assay in which latex bound to an anti-human cystatin C antibody is used to measure the absorption or scattering of visible light (see Patent Document 1) and an enzyme labeling method (see Patent Document 2) are known.
There is also known a method of examining a renal disease, which method is characterized by measuring the concentration of cystatin C excreted into urine and the concentration of an endogenous clearance substance, creatinine, to calculate the ratio of the cystatin C concentration and the creatinine concentration and using the thus obtained data (see Patent Document 3).
Patent Document 4 describes a method of quantifying cystatin C in which a prozone phenomenon inhibitor for measurement of immunological reactions is employed, the inhibitor being characterized by comprising one or more sulfate ester-based and sulfonate-based anionic surfactants.
It has been indicated that cystatin C, because of its physical properties, is likely to adsorb to plastic and glass containers. Since an automatic analyzer often employs a commercially available plastic container, in clinical measurement of cystatin C, it is believed important to prevent a decrease in the measurement accuracy caused by the adsorption of cystatin C to the plastic container. As a method of inhibiting the adsorption of cystatin C to the container, it is thought to subject the container itself to a surface treatment; however, there is a problem in that a surface-treated special container would then be necessary, which results in an increase in the cost.
Therefore, there is a need for a method of accurately and automatically quantifying cystatin C with simple operation, and a means for inhibiting the adsorption of cystatin C to a container in a simple manner is demanded.